An aluminum substrate has been widely used as a substrate for a magnetic recording medium such as a magnetic disk. As the magnetic disks are being produced in smaller sizes and smaller thickness while recording data at a higher density, the aluminum substrate has been gradually replaced by a glass substrate having superior surface flatness and substrate strength. As the glass substrates for magnetic recording medium, there have been used chemically reinforced glass substrates for enhancing the substrate strength and crystallized glass substrates featuring increased substrate strength based on the crystallization.
Accompanying the trend toward high-density recording, further, the magnetic head has been changed from a thin-film head to a magneto-resistive head (MR head) and to a giant magneto-resistive head (GMR head). It is therefore expected that reproducing the contents of the magnetic recording medium of the glass substrate using a magneto-resistive head will become standard in the future.
Thus, a variety of improvements have been made to the magnetic disk for high-density recording. Advances in the magnetic disk, however, has been accompanied by new problems concerning the glass substrate for the magnetic recording medium. One of the problems is to clean the surfaces of the glass substrate. That is, adhesion of a foreign matter to the surfaces of the glass substrate could become a cause of defects in the thin film formed on the surfaces of the glass substrate or a cause of protuberances formed on the surfaces of the thin film. Further, in reproducing the contents the magnetic recording medium of the glass substrate by using a magneto-resistive head, if the flying height (floating height) of the head is lowered to increase the recording density, there may often occur erroneous reproducing operation or a phenomenon in that the reproduction is not accomplished. The cause is that the protuberances formed on the surface of the magnetic disk due to particles on the glass substrate turn into thermal asperity, generating heat in the magneto-resistive head, varying the resistance of the head, and adversely affecting the electromagnetic conversion.
A principal cause of foreign matter on the surface of the glass substrate for the magnetic recording medium described above is that the end surface of the glass substrate is not smooth and, hence, the end surface abrades the wall surface of the resin casing, whereby resin or glass particles formed by the abrasion as well as other particles trapped on the inner peripheral end surface and the outer peripheral end surface of the glass substrate, adhere to the surfaces.
Patent document 1 (JP-A-11-221742) discloses a polishing method wherein a disk-like glass substrate (substrate for a recording medium) having a circular hole at the central portion is immersed in a polishing solution containing free grains, and the inner peripheral end surfaces and/or the outer peripheral end surfaces of the glass substrate are polished by being brought into rotational contact with a polishing brush or a polishing pad while using the polishing solution containing the free grains.
When the inner peripheral end surface of the substrate is to be polished by using the above slurry and the polishing brush, the polishing is effected by rotating the brush while dripping the slurry onto the center hole of the object to be polished which is formed by stacking a plurality of pieces of substrates while aligning the center holes of the disk-like substrates having center holes, or inserting a rod-like polishing brush in the center hole in a state where the object to be polished is immersed in the slurry. The polishing brush has brush hairs studded on the periphery of the rod-like shaft. When a plurality of pieces of substrates are to be simultaneously polished by rotating the polishing brush, there takes place a dispersion in the amount of working on the inner peripheral surface, i.e., the degree of polishing the substrate increases away from the end where the rod is fixed as compared to the inner periphery of the substrate close to the end where the rod is fixed (i.e., as compared to the side that is coupled to the rotary device). This becomes conspicuous with a smaller diameter of the center hole. The above dispersion is considered to stem from the deformation of a thin rod for a substrate having a particularly small center hole due to deviation of the shaft of the rod from the axis of rotation and deflection of the rod when it rotates. Dispersion in the amount of working turns out to be a dimensional error of the product substrate. Therefore, a large dispersion cannot be tolerated.
[Patent document 1] Japanese Unexamined Patent Publication (Kokai) No. 11-221742